1. Field of the Invention
The present invention relates to a polishing method for conducting mirror-polishing of a semiconductor wafer based on a mechano-chemical polishing method and particularly, to a polishing method for a semiconductor wafer in which the semiconductor wafer is mechano-chemically polished by a polishing step for planarization and improvement of surface roughness, wherein a polishing pad in the polishing step is improved and thereby micro-roughness, especially micro-roughness called haze is improved, and a polishing pad used in the polishing method.
2. Description of the Prior Art
Generally, a manufacturing process of a semiconductor wafer comprises a slicing step of slicing a single crystal ingot to obtain a thin disk like wafer, a chamfering step of chamfering the outer periphery of the wafer in order to prevent the wafer obtained in the slicing step from breakage or chipping, a lapping step of flattening the wafer, an etching step of removing a damaged region remained in the surface and edges of the wafer after the chamfering and lapping, a mirror-polishing step of obtaining a mirror surface of the wafer and a cleaning step of cleaning the polished wafer to eliminate attached polishing slurry and other foreign matter from the wafer.
The mirror-polishing step for a wafer uses a polishing pad and aqueous polishing slurry which is produced in such a manner that abrasive particles such as SiO.sub.2 base fine particles or the like are suspended in an aqueous weak alkaline solution, wherein the wafer is polished by means of a so-called mechano-chemical polishing method (a mechanical, chemical composite method). In the mechano-chemical polishing method, a dynamic action of mechanical polishing and a chemical action of etching are combined and a synergetic effect in combination of the actions results in achievement of a mirror surface of a wafer to excellent precision with high efficiency, in which characteristics in polishing of a wafer are largely affected by a balance between a mechanical factor and a chemical factor in polishing.
In other words, in a initial stage of the polishing step, the mechanical factor acts on a wafer so strongly that a high degree of flatness is realized on the surface of a wafer with high efficiency, while in a final stage, the mechanical factor is reduced and mirror-polishing is conducted so that surface roughness is improved, that working damages are eliminated and that excellent precision is attained.
In such a conventional polishing method, polishing is performed in a plurality of stages: for example, in the three stages; polishing goes in combination of first polishing, second polishing and finish polishing.
A conventional polishing method have been conducted as follows: In the first and second polishing stages, colloidal silica is used as abrasive particles in polishing slurry and polishing pad of a non-woven cloth type is used. A relative speed between the wafer and the polishing pad is set to 100 m/min and a pressure on the wafer is set to 500 g/cm.sup.2 with results that a flattening operation is almost completed and a flatness of about 0.3 .mu.m is secured in a final part of the first polishing stage, whereas a surface roughness remains still to be about 1 nm in the final part. Thereafter, the second polishing is conducted while changing a particle size of colloidal silica with results that the wafer of a flatness about 0.3 .mu.m and a surface roughness about 0.5 nm are attained.
Then, finish polishing is conducted in conditions of a standard polishing pressure of 340 g/cm.sup.2 and a low relative speed of 40 m/min using polishing slurry including colloidal silica as in the preceding stages and a polishing pad of a suede type with results of flatness of about 0.3 .mu.m, surface roughness of about 0.35 nm and a haze level of about 50 bits. A surface roughness at this level has a correlation with a haze level of the same specimen which is obtained from measurement of light scattering intensity, whereby a surface condition of a wafer can be evaluated by investigating a haze level thereof.
The applicant of the present invention has provided a two-staged wafer polishing method whereby a quality equal to that obtained by the three-staged polishing method above described can be secured (see the published Unexamined Japanese Patent Application No. Hei 9-38849).
Such a prior art is conducted in such a manner that the plurality of stages in a polishing step includes a first polishing stage and a finish polishing stage. In the course of polishing step, a relative speed and a pressure are changed: in a final part of the first polishing stage, rapid acceleration to a relative speed two to four times as fast as an actual one in the running and rapid reduction to a pressure 1/2 to 1/10 time as high as an actual one in the running; and in a final stage of the finish polishing stage, an actual relative speed in the running is rapidly reduced to a relative speed 1/2 to 1/5 times as fast as the actual one.
In recent years, a trend toward a thinner oxide film in company with progress to higher integration in a device design has been advanced and it has been suggested that the surface roughness exerts an influence on an oxide film breakdown voltage, especially time dependent dielectric breakdown (TDDB) characteristics. Accordingly, while stable supply of wafers each with good surface roughness has become important, there is a fault in the above described polishing conditions that, when a polishing pad of a different production lot is used (when a polishing pad already in use is replaced with a new one still in the same polishing conditions), surface roughness of a wafer is sometimes deteriorated thanks to the replacement. A cause for the deterioration in surface roughness is conceived a change in a physical property of a polishing pad.
Therefore, in order to secure stable supply of wafers each with good surface roughness, it is necessary to effect an incoming inspection of polishing pads, especially polishing pads for use in a finish polishing stage and use only polishing pads each with good physical properties for polishing a silicon wafer.
However, it has, heretofore, not been known what physical properties exert influences on surface roughness and accordingly, it has been difficult to set criteria for an incoming inspection for a polishing pad to be used in finish polishing, which has in turn made stable supply of silicon wafers each with good surface roughness difficult.